The first thing to do on a new to you Max is sync the carbs! If the sync is out it can cause all sorts of havoc with idling and running (not smoothly) through acceleration.
And the good news is: sync takes a whole 5-10mns to do, and you hardly have to take anything off the bike (carb covers). All you need is a sync tool like the Morgan Carbtune ($100ish) or you can make your own for about $15. (Use the search tool)
To me it sounds like your bike was tuned to run with vboost stuck to on, which won't idle right as it's way too rich. In your middle position you prolly have the vboost off, so it'll idle happily but cough away on acceleration being starved.
There's a possibility that switch to left is the OEM setting, and that may work best - no boost at idle, comes on around 6000 rpm. That's the way Yamaha designed it and still the way it works best overall. A lot of people, after installing Tboost (which opens at 3k instead of 6k) took their bikes to a dyno run only to find out they made better power with the boost coming on at 6k..
So, do yourself a favor and sync your carbs first.
Then, with the bike on center stand, warm it up, and try revving it quickly to around 6000rpm while looking at the vboost motor/cable assembly (remove whatever you need to until you can see it using a flashlight if needed). The easiest way to find/see it moving is to switch your ignition off and on - you should hear (and see) it cycle open and close.
This way you can find out what it does depending where your switch is, as noted previously the three positions usually are off, on @ 3000rpm and on @ 6000rpm but find out and let us know, it'll really help on deciding what's best to do!
